Add "Contributing" page to "Community" section

[oruebel]

It would be useful to have a page to point to different ways on how to contribute to NWB, e.g.:

• Contributing to events: Point to the training and event resources page
• Contributing to NWBEP:
  • Point to the reviewer volunteer form
  • Discussing joining working groups
  • Point to resources for making NDX and NWBEP
• Source Code: Port general information from this document https://pynwb.readthedocs.io/en/stable/contributing.html and point to the individual repos for details